Internet DRAFT - draft-vixie-dns-rpz
draft-vixie-dns-rpz
Domain Name System Operations P. Vixie
Internet-Draft Farsight Security, Inc.
Intended status: Informational V. Schryver
Expires: June 19, 2017 Rhyolite Software, LLC
December 16, 2016
DNS Response Policy Zones (RPZ)
draft-vixie-dns-rpz-04
Abstract
This document describes a method for expressing DNS response policy
inside a specially constructed DNS zone, and for recursive name
servers to use such policy to return modified results to DNS clients.
The modified DNS results can stop access to selected HTTP servers,
redirect users to "walled gardens", block objectionable email, and
otherwise defend against attack. These "DNS Firewalls" are widely
used in fighting Internet crime and abuse.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 19, 2017.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may not be modified, and derivative works of it may not
be created, except to format it for publication as an RFC or to
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Discussion Venue . . . . . . . . . . . . . . . . . . . . 3
2. Zone Format . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Policy Actions . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. The "NXDOMAIN" Action (CNAME .) . . . . . . . . . . . . . 5
3.2. The "NODATA" Action (CNAME *.) . . . . . . . . . . . . . 5
3.3. The "PASSTHRU" Action (CNAME rpz-passthru.) . . . . . . . 5
3.4. The "DROP" Action (CNAME rpz-drop.) . . . . . . . . . . . 6
3.5. The "TCP-Only" Action (CNAME rpz-tcp-only.) . . . . . . . 6
3.6. The "Local Data" Action (arbitrary RR types) . . . . . . 6
4. Policy Triggers . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. The "Client IP Address" Trigger (.rpz-client-ip) . . . . 9
4.2. The "QNAME" Trigger ("example.com") . . . . . . . . . . . 10
4.3. The "Response IP Address" Trigger (.rpz-ip) . . . . . . . 10
4.4. The "NSDNAME" Trigger (.rpz-nsdname) . . . . . . . . . . 11
4.5. The "NSIP" Trigger (.rpz-nsip) . . . . . . . . . . . . . 12
5. Precedence Rules . . . . . . . . . . . . . . . . . . . . . . 13
5.1. "CNAME or DNAME Chain Position" Precedence Rule . . . . . 14
5.2. "RPZ Ordering" Precedence Rule . . . . . . . . . . . . . 14
5.3. "Domain Name Matching" Precedence Rule . . . . . . . . . 14
5.4. "Trigger Type" Precedence Rule . . . . . . . . . . . . . 15
5.5. "Name Order" Precedence Rule . . . . . . . . . . . . . . 15
5.6. "Prefix Length" Precedence Rule . . . . . . . . . . . . . 16
5.7. "IP Address Order" Precedence Rule . . . . . . . . . . . 16
6. Application of the Policy . . . . . . . . . . . . . . . . . . 17
6.1. Per-Zone Action Overrides . . . . . . . . . . . . . . . . 18
7. Producer Behavior . . . . . . . . . . . . . . . . . . . . . . 19
8. Subscriber Behavior . . . . . . . . . . . . . . . . . . . . . 20
9. Implementation Considerations . . . . . . . . . . . . . . . . 20
9.1. Avoid Waiting for QNAME Recursion . . . . . . . . . . . . 21
9.2. Check Parents Domains versus Zone Cuts . . . . . . . . . 21
9.3. Abbreviate the Data Path for NSDNAME and NSIP Checks . . 22
9.4. Use Glue for NSDNAME and NSIP Checks . . . . . . . . . . 22
9.5. Reduce Zone Size using Implied Rules . . . . . . . . . . 23
10. History and Evolution . . . . . . . . . . . . . . . . . . . . 23
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
12. Security Considerations . . . . . . . . . . . . . . . . . . . 25
12.1. DNS Data Security . . . . . . . . . . . . . . . . . . . 25
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12.2. DNSSEC . . . . . . . . . . . . . . . . . . . . . . . . . 25
12.3. TSIG . . . . . . . . . . . . . . . . . . . . . . . . . . 25
12.4. Counterintelligence . . . . . . . . . . . . . . . . . . 25
12.5. NSIP, NSDNAME, Glue, and Authoritative RRsets . . . . . 26
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 26
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
14.1. Normative References . . . . . . . . . . . . . . . . . . 26
14.2. Informative References . . . . . . . . . . . . . . . . . 27
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30
1. Introduction
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This document describes DNS Firewalls, a method of expressing DNS
[RFC1034] response policy information inside specially constructed
DNS zones, known as Response Policy Zones (RPZs). RPZs allow
Internet security policy producers and subscribers to cooperate in
the application of policies to modify DNS responses in real time.
Using the policy information, DNS resolution for potentially unsafe
DNS data can be made to deliberately fail or to return local data
such as an alias to a "walled garden".
A site's DNS response policy consists of the set of rules expressed
in all of the RPZs that it uses. Each rule, expressed as an RRset,
consists of a trigger (left hand side or owner name) and an action
(right hand side). A full description of the expressible policies is
given in Section 3 (actions) and Section 4 (triggers).
Configuration examples are given using ISC BIND Version 9 (BIND9)
[ISC-ARM] syntax, because work to add RPZ to that platform was
started earliest (in 2009). The RPZ specification itself is free to
implement and free to use in operation. It has been implemented in
other name server software. We expect that in time, additional
recursive DNS implementations will also implement DNS Firewalls as
described by this RPZ specification.
1.1. Discussion Venue
The discussion venue for this document is the DNS Firewalls mailing
list. http://lists.redbarn.org/mailman/listinfo/dnsfirewalls offers
subscriptions and archives. See also https://dnsrpz.info/
[NOTE TO EDITOR: This section must be removed before this Internet
Draft is published as an RFC.]
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2. Zone Format
A DNS Response Policy Zone (RPZ) is a DNS zone. Like any DNS zone,
an RPZ consists of RRsets or sets of resource records (RRs) with a
common owner name and type. RRsets other than SOA, NS, and DNSSEC-
related [RFC4034] specify actions and triggers. The owner name (left
hand side) of each RRset expresses a policy trigger, while the RDATA
(right hand side) encodes the action to be taken when the trigger
matches. Depending on the type of trigger (see Section 4), a
particular characteristic of the DNS response is checked.
Characteristics that can be checked include the domain name (QNAME),
the IP address of the querying client, IP addresses or domain names
in the answer section of the response, and authoritative name server
names and addresses.
Because an RPZ is a valid DNS zone, its contents can be transferred
between DNS servers in whole (AXFR) [RFC5936] or incrementally as
changes occur (IXFR) [RFC1995], can be authenticated and protected by
TSIG transaction signatures [RFC2845], and can have update
propagation expedited by real time change notifications (NOTIFY)
[RFC1996], all subject to familiar DNS access controls. An RPZ need
not support query access since query access is never required. It is
the zone transfer of RPZ content from producers to subscribers which
effectively publishes the policy data. However, it is the
subscribers' configurations of their recursive name servers which
promote RPZ payload data into the control plane data of their
individual name servers.
Every valid DNS zone including an RPZ has an SOA record and at least
one NS record at its apex, which is why the SOA and NS records of an
RPZ cannot themselves be used to encode DNS response policy rules.
The RPZ's SOA record is real, with a serial number used for NOTIFY
and IXFR, and timers used for AXFR and IXFR. The MNAME field or
domain name of the primary source of the zone and the RNAME field or
mailbox of the person responsible for the zone SHOULD be used by RPZ
providers to label their policy zones.
The apex NS RRset will never be used since RPZ does not rely on query
access. Similarly, no parent delegation is required for normal
operation of the RPZ. Thus, by convention, a single NS record having
the deliberately bogus RDATA of "LOCALHOST." is used as a
placeholder.
While loading an RPZ, any data which is not syntactically valid for
DNS should cause normal error processing as would occur for any DNS
zone. If an RR found in an RPZ is meaningless or unusable for
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response policy purposes, then the containing RRset SHOULD be
ignored, and an error MAY be logged.
The format of RPZs has undergone several revisions since work began
(see Section 10, History and Evolution). All policy triggers and
actions described here are valid as of Format 3. Policy triggers
from a higher format number than a recursive name server's
implementation level are expected to be harmless to that
implementation. Policy actions from a higher format number are
likely to be misinterpreted as CNAME Local Data by older
implementations. When possible, implementations SHOULD treat policy
triggers or actions of higher format numbers as they treat other
errors, as described above.
3. Policy Actions
An RPZ resource record can specify any of six results or actions.
3.1. The "NXDOMAIN" Action (CNAME .)
A single resource record (RR) consisting of a CNAME whose target is
the root domain (.) will cause a response of NXDOMAIN to be returned.
This is the most commonly used RPZ action.
$ORIGIN RPZ.EXAMPLE.ORG.
example.com CNAME . ; return NXDOMAIN
*.example.com CNAME . ; return NXDOMAIN
3.2. The "NODATA" Action (CNAME *.)
A single RR consisting of a CNAME whose target is the wildcard top-
level domain (*.) will cause a response of NODATA (ANCOUNT=0) to be
returned regardless of query type.
$ORIGIN RPZ.EXAMPLE.ORG.
example.com CNAME *. ; return NODATA
*.example.com CNAME *. ; return NODATA
3.3. The "PASSTHRU" Action (CNAME rpz-passthru.)
It is sometimes necessary to exempt some DNS responses from a policy
rule that covers an entire domain or a large IP address block.
Exempting some clients of a DNS resolver from all RPZ rewriting can
also be useful for research into attackers and for debugging. The
PASSTHRU action is intended to override other, usually more general
policies. The trigger for the PASSTHRU action MUST have a higher
precedence than the policies that it should override (see Section 5,
Precedence Rules).
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In the example below, the first PASSTHRU record exempts requests for
a particular host from the NXDOMAIN policy action of the subsequent
records. The second PASSTHRU record exempts responses to the DNS
client at 192.0.2.1 from being modified:
$ORIGIN RPZ.EXAMPLE.ORG.
ok.example.com CNAME rpz-passthru.
32.1.2.0.192.rpz-client-ip CNAME rpz-passthru.
example.com CNAME .
*.example.com CNAME .
3.4. The "DROP" Action (CNAME rpz-drop.)
The DROP policy that results in discarding the request and response
is specified by a CNAME whose target is "rpz-drop". For example, the
following rule mandates that nothing be sent to a DNS client trying
to resolve "EXAMPLE.COM", not even a DNS error response:
$ORIGIN RPZ.EXAMPLE.ORG.
example.com CNAME rpz-drop.
3.5. The "TCP-Only" Action (CNAME rpz-tcp-only.)
The TCP-Only action is specified by a CNAME whose target is
"rpz-tcp-only". It changes UDP responses to short, truncated DNS
responses that require the DNS client to try again with TCP. It is
used to mitigate distributed DNS reflection attacks and is similar to
the "slip" parameter of DNS Response Rate Limiting (RRL) [ISC-RRL].
$ORIGIN RPZ.EXAMPLE.ORG.
example.com CNAME rpz-tcp-only.
3.6. The "Local Data" Action (arbitrary RR types)
A set of RRsets with a common trigger owner name (see Section 4) that
includes neither a special CNAME RPZ encoding of an action nor one of
the problematic record types listed below specifies data to be used
to generate synthetic DNS responses. The most common Local Data is a
CNAME RR pointing to a walled garden, although other record types are
also used.
$ORIGIN RPZ.EXAMPLE.ORG.
bad1.example.com CNAME garden.example.net.
bad2.example.com A garden-web.example.net.
bad2.example.com MX garden-mail.example.net.
32.3.2.0.192.rpz-client-ip A quarantine.example.net.
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Note that because an RPZ is a valid DNS zone, if the action of a
policy rule contains a CNAME RR, then no other RRs are allowed for
that owner name (trigger).
The special RPZ encodings which are not to be taken as Local Data are
CNAMEs with targets that are:
+ "." (NXDOMAIN action),
+ "*." (NODATA action),
+ a top level domain starting with "rpz-",
+ a child of a top level domain starting with "rpz-".
The problematic types and records which also do not encode Local Data
actions include:
+ SOA records,
+ NS records,
+ DNAME records,
+ all DNSSEC-related records (see [RFC4034]).
RRsets of problematic record types MUST NOT be included in RPZ data,
because their appearance in responses would be invalid or confuse DNS
clients.
When a Local Data policy rule matches, the RRsets of Local Data are
used to generate the response as if they comprised all of the
authoritative data for the QNAME. If the requested type (QTYPE) is
ANY, then all of these Local Data RRsets are returned. Otherwise,
the RRset of the requested RR type is returned, or a CNAME is
returned if it is available. If no CNAME nor RRset of the requested
type is available, then the response is normally NODATA (ANCOUNT=0).
Using the example above, if client 192.0.2.3 asks for MX records, it
will receive NODATA, because the policy rule with the matching Client
IP Address trigger contains RRsets of RRtype A but none of RRtype MX.
This normal NODATA response when there are no Local Data records of
the requested type can be changed with the LOCAL-DATA-OR-PASSTHRU or
LOCAL-DATA-OR-DISABLED overrides described in Section 6.1.
A special form of Local Data involves a CNAME RR with a wildcarded
target name. Wildcards are not valid as CNAME targets in ordinary
DNS zones. However, a wildcard in an RPZ Local Data CNAME target
causes the matching QNAME to be prepended to the target in the
rewritten response, which communicates this QNAME value to the walled
garden DNS server for that DNS server's logs.
For example a policy Local Data action of "CNAME *.EXAMPLE.COM"
applied to a QNAME of "EVIL.EXAMPLE.ORG." will result in a synthetic
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response that starts with the RR
"EVIL.EXAMPLE.ORG CNAME EVIL.EXAMPLE.ORG.EXAMPLE.COM". Resolving the
CNAME target "EVIL.EXAMPLE.ORG.EXAMPLE.COM" into an RRset of the
originally requested type generally requires sending a request for
that type and a QNAME of "EVIL.EXAMPLE.ORG.EXAMPLE.COM" to a DNS
server for the walled garden, "EXAMPLE.COM". As usual when a CNAME
is encountered while computing a response, the response from the
walled garden DNS server concerning "EVIL.EXAMPLE.ORG.EXAMPLE.COM"
determines the rest of the final rewritten response.
In the example below, a client that asks for A RRs for
"BAD.EXAMPLE.COM" will receive a response starting with
"BAD.EXAMPLE.COM CNAME BAD.EXAMPLE.COM.GARDEN.EXAMPLE.NET". The DNS
server using RPZ will then probably try to resolve
"BAD.EXAMPLE.COM.GARDEN.EXAMPLE.NET" by requesting A RRs from the
authority for "GARDEN.EXAMPLE.NET". That authority should answer
with NODATA, NXDOMAIN, or an A RRset, but in any case can log the
request to show that a request for "BAD.EXAMPLE.COM" has been
received.
$ORIGIN RPZ.EXAMPLE.ORG.
bad.example.com CNAME *.garden.example.net.
4. Policy Triggers
There are five types of RPZ triggers, and they are encoded by RRset
owner names (left hand sides) in an RPZ.
Two of these trigger types match characteristics of the DNS query:
Client IP Address and QNAME. While these two trigger typess are
independent of cache contents or recursion results, still
conceptually they must be checked only once the response is ready,
because they compete for precedence (see Section 5) with other
trigger types which depend on what happens during recursion.
The other three trigger types are based on characteristics of the DNS
response, that is, on the RDATA to be returned to the client, or in
some cases, on NS-related RDATA used while recursively obtaining the
final response, regardless of whether or not those NS or NS-related
records are themselves to be returned to the client. These three
trigger types are: Response IP Address, NSDNAME, and NSIP.
Neither the TTL fields of RRs in the answer section nor the query
type (QTYPE) in the question section of responses are used as RPZ
triggers, although the QTYPE is used to select appropriate RRsets
among the RRsets of matching Local Data rules (Section 3.6).
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All policies are conceptually applied after recursion, even though in
practice recursion can sometimes be skipped, if doing so would not
change the RPZ result (see Section 5, Precedence Rules and Section 9,
Implementation Considerations). As a result, the recursive DNS
resolver's cache contains either nothing or "truth", even if this
truth is hidden by current policy. If the policy changes, the
original, unmodified data is available for processing under the
changed policy.
4.1. The "Client IP Address" Trigger (.rpz-client-ip)
The IP addresses of DNS clients sending requests can be used as
triggers, which can be useful for disabling RPZ rewriting for DNS
clients used for testing or investigating, or for quarantining
compromised clients. Client IP Address policy RRsets have owner
names that are subdomains of "rpz-client-ip" relativized to the RPZ
apex name, preceded by an encoded IP address or block of addresses.
For example, the following would drop all requests from clients in
192.0.2.0/24 and give truthful answers to requests from a client at
2001:db8::3.
$ORIGIN RPZ.EXAMPLE.ORG.
24.0.2.0.192.rpz-client-ip CNAME rpz-drop.
128.3.zz.db8.2001.rpz-client-ip CNAME rpz-passthru.
4.1.1. IP address encoding in triggers
The IPv4 address or address block "B1.B2.B3.B4/prefix" is encoded in
an RPZ trigger as "prefix.B4.B3.B2.B1", with the address octets
reversed just as in the IN-ADDR.ARPA naming convention described in
[RFC1034]. The prefix length ("prefix") is a decimal integer from 1
to 32. All four octets, B4, B3, B2, and B1, must be present and are
also decimal integers. To avoid confusion with octal notation,
leading zeros MUST be suppressed.
For example, the address block 192.0.2.0/24 is encoded as
"24.0.2.0.192".
The IPv6 address or address block beginning at
"W1:W2:W3:W4:W5:W6:W7:W8" is encoded in a format similar to the
standard IPv6 text representation (see [RFC5952]), again reversed:
"prefix.W8.W7.W6.W5.W4.W3.W2.W1". The prefix length ("prefix") is a
decimal integer from 1 to 128. Each of W8,...,W1 is a one- to four-
digit hexadecimal number representing 16 bits of the IPv6 address.
As in [RFC5952], in order to avoid confusion with octal notation,
leading zeroes MUST be suppressed.
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All 8 hextets must be present unless a "zz" label is present. The
"zz" label is analogous to the double-colon (::) in [RFC5952], and it
is required and allowed just as the double-colon is required and
allowed in that document. In particular, the longest possible
sequence of zero-valued fields MUST be compressed to "zz". If there
exists more than one sequence of zero-valued fields of identical
length, then only the last such sequence is compressed. Note that
[RFC5952] specifies compressing the first such sequence, but our
notation here reverses the order of fields, and so must also reverse
the selection of which zero sequence to compress.
For example, the address 2001:db8::3 with a prefix length of 128
would be encoded as "128.3.zz.db8.2001".
In the case of an address block, either IPv4 or IPv6, the address
part MUST NOT contain any non-zero bits after the section indicated
by the prefix length. For example, "8.2.0.0.10.rpz-client-ip" is not
a valid trigger, because in the address "10.0.0.2" expressed in
binary notation, a one occurs outside the first 8 bits or prefix of
the address block.
4.2. The "QNAME" Trigger ("example.com")
The QNAME policy trigger matches requested domains, that is, the
QNAME field of the question sections in DNS requests and responses.
(See [RFC1035].) The owner name of an RPZ QNAME policy RRset is the
relativized name of the domain name about which policy is being
expressed. For example, if the RPZ apex name is "RPZ.EXAMPLE.ORG",
an RRset at "EXAMPLE.COM.RPZ.EXAMPLE.ORG" would affect responses to
requests about "EXAMPLE.COM".
Wildcards work as expected, so the owner name
"*.EXAMPLE.COM.RPZ.EXAMPLE.ORG" would match queries for any subdomain
of "EXAMPLE.COM". To control the policy for both a name and its
subdomains, two policy RRsets must be used, one for the domain itself
and another for a wildcard subdomain. In the following example,
queries for both "EXAMPLE.COM" and all subdomains of "EXAMPLE.COM"
will result in synthetic NXDOMAIN responses.
$ORIGIN RPZ.EXAMPLE.ORG.
example.com CNAME .
*.example.com CNAME .
4.3. The "Response IP Address" Trigger (.rpz-ip)
The Response IP Address trigger matches IP addresses that would
appear in the unaltered DNS response contents, specifically the RDATA
of A or AAAA records in the answer sections of DNS responses. IP
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addresses in the authority and additional sections are not
considered. Response IP Address policy RRsets have owner names that
are subdomains of "rpz-ip" relativized to the RPZ apex name, and an
encoded IP address or block of addresses. The IP address encodings
are identical to those described in Section 4.1.1 for Client IP
Address triggers.
For example, to force an NXDOMAIN response whenever a truthful
response would contain an answer section A RRset having an address in
192.0.2.0/24 unless address 192.0.2.2 is present, the RPZ would
contain these records:
$ORIGIN RPZ.EXAMPLE.ORG.
24.0.2.0.192.rpz-ip CNAME .
32.2.2.0.192.rpz-ip CNAME rpz-passthru.
In another example, to answer NODATA (ANCOUNT=0) whenever a truthful
response would contain an answer AAAA RRset having an address
2001:db8:101::/48 unless address 2001:db8:101::3 was present, the RPZ
would contain these records:
$ORIGIN RPZ.EXAMPLE.ORG.
48.zz.101.db8.2001.rpz-ip CNAME *.
128.3.zz.101:db8.2001.rpz-ip CNAME rpz-passthru.
Please refer to Section 5 (Precedence Rules) to understand how the
above exception mechanisms work.
4.4. The "NSDNAME" Trigger (.rpz-nsdname)
The NSDNAME policy trigger matches name server names (NS RR) of all
name servers in the data paths for all RRsets that would be present
in the answer section of the unaltered DNS response.
The data path for a given answer RRset consists of all delegation
points from (and including) the root zone down to the closest
enclosing NS RRset for the owner name of that RRset. Names in the
RDATA of answer RRs including CNAME, DNAME, SRV, and MX are not
themselves directly relevant, but CNAME and DNAME target names are
indirectly relevant if they cause RRsets to be added to the answer
section, in which case it is the data paths of the added RRsets that
matter. In the case of a DNAME answer, the owner name of an added
synthetic CNAME is likely to differ from the target name in the DNAME
RR. Recall also that the target of a CNAME is not added to the
response if the QTYPE is ANY or CNAME or if this target cannot be
resolved (e.g. NXDOMAIN or SERVFAIL errors).
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NSDNAME policies are encoded as RRsets in subdomains of "rpz-nsdname"
but otherwise are much like QNAME policies (Section 4.2). For
example, to force an NXDOMAIN answer whenever a name server for the
requested domain or one of its parents is "NS.EXAMPLE.COM", the RPZ
would contain the following:
$ORIGIN RPZ.EXAMPLE.ORG.
ns.example.com.rpz-nsdname CNAME .
The NS records used for this calculation are either delegations (NS
RRs in the authority sections of answers from authorities for the
parent zone) or authoritative data from the zone itself. An
implementation MAY use either, both, or whichever is currently
available. See Section 9.4 about some implementation considerations
for this choice, and Section 12.5 about security considerations.
An RPZ implementation MAY be configurable to avoid checking all the
way up to the root and to perform only partial NSDNAME checks; see
Section 9.3 on "min-ns-dots".
4.5. The "NSIP" Trigger (.rpz-nsip)
The NSIP policy trigger matches name server addresses, that is A or
AAAA RRs referenced by an NS RRset. NSIP is like NSDNAME
(Section 4.4) except that the matching is by name server address
rather than name server name. NSIP policies are expressed as
subdomains of "rpz-nsip" and have the same subdomain naming
convention as that described for encoding IP addresses in Response IP
Address triggers (Section 4.1.1).
In a process similar to that for an NSDNAME trigger, an NSIP trigger
is checked by considering all of the IP addresses for all of the name
servers in the data paths for all RRsets that would be present in the
answer section of the unaltered DNS response.
As with NSDNAME triggers, the data path for a given RRset consists of
all delegation points from (and including) the root zone down to the
closest enclosing NS RRset for the owner name of that RRset. Also
like NSDNAME triggers, the RDATA in these RRsets (other than the IP
addresses of the name servers) are not directly relevant.
For example, to force an NXDOMAIN answer whenever one of the name
servers for the requested domain (QNAME) or one of its ancestors has
an address in the 192.0.2.0/24 block, the RPZ would contain the
following:
$ORIGIN RPZ.EXAMPLE.ORG.
24.0.2.0.192.rpz-nsip CNAME .
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The NS, A, and AAAA records used for this calculation are either
delegations and glue (RRs in authority and additional sections of
answers from authorities for the parent zone) or authoritative data
from the zone itself. As with NSIP, an implementation MAY use
either, both, or whichever is currently available; see Section 9.4 on
"ns-wait-recurse".
An RPZ implementation MAY also be configurable to avoid checking all
the way up to the root and to perform only partial NSIP checks; see
Section 9.3 on "min-ns-dots".
5. Precedence Rules
More than one policy trigger among the various DNS RPZs connected to
the name server's control plane can match while computing a given DNS
response, but only a single policy rule can rewrite the response.
The policy rule with the best match will be selected according to the
precedence rules outlined below.
These precedence rules exist and are ordered to ensure that RPZ
subscribers and publishers have the same understanding of what a set
of policy zones will do, and to ensure that subscribers can use local
zones to override published policy zones.
In theory and for standardization, all matching policy rules are
considered simultaneously, and the precedence rules are used to
choose the single best RPZ rule. In actual implementations, policy
triggers are usually considered in a sequence that mirrors the
process of generating the DNS response, because checking RPZ triggers
is conveniently made a part of that process. For example, Client IP
Address triggers are often checked early as the DNS request is being
received and as the client IP address is being checked in the access
control list (ACL) that determines which DNS client IP addresses can
ask for recursion. Likewise the QNAME is available for RPZ trigger
matching before any response IP addresses are known, so QNAME
triggers are usually checked immediately after Client IP Address
triggers and before Response IP Address triggers. NSIP and NSDNAME
triggers are generally checked last.
As far as the DNS client can determine, it MUST seem that all
matching triggers are found and weighed using these precedence rules,
even though in practice, shortcuts are taken. Shortcuts MUST NOT
affect the outcome. For example, according to the precedence rules,
a matched QNAME trigger in the first policy zone makes all Response
IP Address, NSIP, and NSDNAME triggers moot. There is no need to
look for those matches, because they cannot further affect the
response.
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The actions specified in policy rules are not used in the calculation
of precedence. The actions of policy rules determined by per-zone
action overrides (Section 6.1), other than those that disable them,
likewise do not affect the calculation of precedence. Override
actions which disable policy rules affect this calculation only by
removing those rules from consideration.
Precedence rules are applied in the order listed here; the comparison
between two matching policy rules to choose the better match is
determined by the first dispositive precedence rule in this list.
Note however that if the best match selects a disabled rule (caused
by the DISABLED override described in Section 6.1), then the next
best match is used, and so on until a policy rule that is not
disabled is selected or no matches remain.
5.1. "CNAME or DNAME Chain Position" Precedence Rule
This precedence rule applies only when the original query type is not
ANY, CNAME, nor DNAME, and only when a CNAME or DNAME is encountered
while resolving the original QNAME, in what we will call the first
"stage of resolution". In this case, we know that the recursive name
server ([RFC1035]) may follow the CNAME (or the synthetic CNAME
constructed while applying a DNAME) by starting a second stage of
resolution starting at this CNAME's target. Additional stages of
resolution may ensue if further CNAMEs or DNAMEs are encountered,
until the final response is computed.
A policy rule match which occurs at an earlier stage of resolution is
preferred to a policy rule match which occurs at a later stage.
Recall that only one policy rule, from among all those matched at all
stages of resolving a CNAME or DNAME chain, can affect the final
response; this is true even if the selected rule has a PASSTHRU
action.
5.2. "RPZ Ordering" Precedence Rule
This precedence rule applies when the matches being compared refer to
policy rules in different RPZs.
Matches on rules in an RPZ specified earlier in the ordered list of
RPZs take precedence over matches on rules in an RPZ specified later.
5.3. "Domain Name Matching" Precedence Rule
This precedence rules applies to a set of QNAME rules or a set of
NSDNAME rules in a single RPZ. The rule triggers are compared.
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This rule is the same as that used by DNS servers to choose the RRs
with which to respond to a request. In particular, an exact name
match is better than one involving a wildcard, and among wildcard
matches, the trigger (owner domain name) that has the largest number
of labels is best.
Since a DNS response has only one QNAME in a given stage of CNAME or
DNAME chain resolution (as defined in Section 5.1,
"CNAME or DNAME Chain Position" Precedence Rule), this precedence
rule is sufficient to choose a single QNAME trigger over other QNAME
triggers in the same RPZ. However, computing a single DNS response
can match dozens of NSDNAME triggers in a single RPZ, of which
several may be equal according to this precedence rule. In that
case, the "Name Order" Precedence Rule (Section 5.5) selects the
single winner from among those chosen by this rule.
5.4. "Trigger Type" Precedence Rule
This precedence rule applies to policy rules within the same RPZ, but
with different trigger types.
In this case, matches are ranked according to trigger type in the
following order, from highest to lowest precedence:
+ Client IP Address
+ QNAME
+ Response IP Address
+ NSDNAME
+ NSIP
5.5. "Name Order" Precedence Rule
This precedence rule applies to a set of NSDNAME matches whose
precedence has not been established by previous precedence rules.
Here, the matched name server domain names are compared, not the
owner names (triggers) of the policy rules. Therefore, it is
irrelevant whether the matched trigger was a wildcard or a specific
domain name.
Among the matching NSDNAME policy rules, choose the one whose matched
name server domain name appears last in the DNSSEC canonical DNS name
order described in section 6.1 of [RFC4034].
Important note: if this precedence rule is reached, the matches being
compared originate from different NS names, not from the same name
matching multiple rules, as those conflicts would have been dispensed
with by the "Domain Name Matching" Precedence Rule (Section 5.3).
There is no a priori reason to prefer one NS name over another, so
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while this tie-breaker precedence rule is dispositive, it is also
arbitrary. All RPZ implementations MUST make the same choices in
these cases to ensure consistent, predictable results among
installations.
Taking part of the example directly from section 6.1 of [RFC4034],
and reversing the order to take into account that the name that
appears last in the DNSSEC canonical order is to be taken as the best
match here, the following NS names are given below in order from
highest or best to lowest precedence:
+ z.example
+ zABC.a.EXAMPLE
+ Z.a.example
+ yljkjljk.a.example
+ a.example
+ example
5.6. "Prefix Length" Precedence Rule
This precedence rule applies when more than one Response IP Address
policy rule or more than one NSIP policy rule matches while computing
a response. The rule triggers themselves are compared.
When comparing two Response IP Address matches or two NSIP matches on
rules within a single RPZ, choose the match whose rule trigger has
the longest "internal prefix length". The internal prefix length of
an IPv6 address trigger is the numeric value of the first label that
defines it as described in Section 4.1.1 on
IP address encoding in triggers. For an IPv4 address trigger, the
internal prefix length is the numeric value of its first label plus
96 (that is, 128-32). This adjustment allows IPv4 and IPv6 addresses
to use the same internal representation, with 32-bit IPv4 addresses
expanded to 128 bits by zero filling as described in section 2.5.5.1
of [RFC4291].
5.7. "IP Address Order" Precedence Rule
This precedence rule applies when more than one Response IP Address
policy rule or more than one NSIP policy rule matches while computing
a response, and they have the same internal prefix length. The rule
triggers are compared.
When comparing Response IP Address or NSIP triggers with equal
internal prefix lengths, choose the trigger which encodes the smaller
IP address. The IP addresses from the triggers are compared as 128
bit numbers, with IPv4 addresses expanded to 128 bits by zero filling
as described in section 2.5.5.1 of [RFC4291].
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Important note: if this precedence rule is reached, the matches being
compared originate from different IP addresses, not from the same
address matching multiple rules, as those conflicts would have been
dispensed with by the "Prefix Length" Precedence Rule (Section 5.6).
There is no a priori reason to prefer one IP address over another, so
while this tie-breaker precedence rule is dispositive, it is also
arbitrary. All RPZ implementations MUST make the same choices in
these cases to ensure consistent, predictable results among
installations.
In the following example, the internal prefix length of all three IP
addresses is 121 as described in Section 5.6.
25.0.2.0.192.rpz-ip CNAME most.example.com.
25.128.2.0.192.rpz-ip CNAME middle.example.com.
121.280.c000.zz.db8.2001.rpz-ip CNAME least.example.com.
The three addresses above are compared as these 128-bit hexadecimal
numbers, respectively:
0x000000000000000000000000c0000200,
0x000000000000000000000000c0000280, and
0x20010db80000000000000000c0000280.
Thus, the first IPv4 address is most preferred and the IPv6 address
is least preferred.
6. Application of the Policy
To enable the use of RPZs, the recursive name server's control plane
is connected to the DNS RPZ data plane by supplying an ordered list
of RPZs in the name server's configuration.
By default, policies are applied only on DNS requests that ask for
recursion (RD=1). Recursive DNS servers generally send their
requests to authority servers without asking for recursion (RD=0),
while stub resolvers ask for recursion (RD=1). Thus, the use of RPZ
at a recursive server by default affects requests from stub resolvers
only. An implementation SHOULD include a configuration option such
as "recursive-only no" to relax this restriction.
Also by default, RPZ policies are applied only while responding to
DNS requests that do not request DNSSEC metadata (DO=0) or for which
no DNSSEC metadata exists. An implementation MAY include a
configuration option such as "break-dnssec yes" to relax this
restriction. See Section 12.2 about the implications of responding
with modified DNS responses when the DNS client seems to be expecting
DNSSEC signatures.
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RPZ rules do not apply to synthetic data generated by using RPZ
rules. For example, if RPZ supplies a CNAME pointing to a walled
garden, RPZ policies will not be used while following that CNAME. If
RPZ supplies local data giving a particular A record, RPZ policies
will not apply to that response IP address.
If an illegal record is encountered while computing a response, for
example an NS record with a CNAME target, and this illegal record is
ignored by the resolver, then it is likewise not used for RPZ rule
matches.
If a policy rule matches and results in a modified answer, then that
modified answer will include in its additional section the SOA RR of
the policy zone whose rule was used to generate the modified answer.
This SOA RR includes the name of the DNS RPZ and the serial number of
the policy data which was connected to the DNS control plane when the
answer was modified.
6.1. Per-Zone Action Overrides
For each DNS RPZ configured for use by a recursive name server, it
SHOULD be possible to specify a single, OPTIONAL overriding policy
action to be used for all policy triggers found in that RPZ. These
policy action overrides include the following:
NXDOMAIN
SHOULD be implemented and is the same as the NXDOMAIN action
defined in Section 3.1.
NODATA
SHOULD be implemented and is the same as the NODATA action defined
in Section 3.2.
PASSTHRU
SHOULD be implemented and is the same as the PASSTHRU action
defined in Section 3.3.
LOCAL-DATA-OR-PASSTHRU
MAY be implemented. It modifies the Local Data action
(Section 3.6) so that requests for RR types that would otherwise
give a Local Data result of NODATA (ANCOUNT=0) are instead handled
using the PASSTHRU action (Section 3.3). Note that computing a
DNS response for QTYPE ANY can never cause a Local Data NODATA
result. The LOCAL-DATA-OR-PASSTHRU override applies only to Local
Data rules and has no effect on rules with other actions.
DROP
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SHOULD be implemented and is the same as the DROP action defined
in Section 3.4.
TCP-ONLY
SHOULD be implemented and is the same as the TCP-ONLY action
defined in Section 3.5.
CNAME domain
SHOULD be implemented and is a special case of the Local Data
action defined in Section 3.6. The overriding data is a CNAME RR
with the given domain as its target.
GIVEN
MAY be implemented to explicitly affirm the default, which is to
respect all policy actions found in this DNS RPZ.
DISABLED
SHOULD be implemented and causes any rule of the zone, when
selected as a "best match", to have no effect, except to log what
would otherwise have happened, provided sufficient logging is
enabled. The DISABLED override thus causes the next highest
precedence match (if any) to be used (see Section 5,
Precedence Rules). This is useful for debugging or previewing a
policy zone.
LOCAL-DATA-OR-DISABLED
MAY be implemented. It modifies the Local Data action
(Section 3.6) so that requests for RR types that would otherwise
give a Local Data result of NODATA (ANCOUNT=0) are instead handled
similarly to the DISABLED override, but without logging. In this
case, the next highest precedence match (if any) is used. Note
that computing a DNS response for QTYPE ANY can never cause a
Local Data NODATA result. The LOCAL-DATA-OR-DISABLED override
applies only to Local Data rules and has no effect on rules with
other actions.
7. Producer Behavior
A DNS RPZ producer SHOULD make every effort to ensure that
incremental zone transfers (IXFR [RFC1995]) rather than full zone
transfers (AXFR [RFC5936]) are used to move new policy data toward
subscribers. DNS RPZ subscribers are "stealth slaves" as described
in RFC 1996. The master MUST allow each such slave to perform the
needed zone transfers, for example via its access control lists
(ACLs). The master also SHOULD be configured as necessary to send
NOTIFY messages to each slave. Because minimal data latency is
critical both to the prevention of crime and abuse and to the
withdrawal of erroneous or outdated policy, a DNS RPZ producer SHOULD
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also make every effort to minimize data latency, including ensuring
that NOTIFY messages are sent in a timely manner after each change of
the DNS RPZ on the master server.
In a response policy zone domain, each addition, deletion, or
expiration could be handled using DNS UPDATE [RFC2136] to trigger
normal DNS NOTIFY and subsequent DNS IXFR activity which can keep the
subscribing servers well synchronized to the master RPZ.
Alternatively, on some primary name servers (such as ISC BIND9) it is
possible to generate an entirely new primary RPZ file and have the
server compute the differences between each new version and its
predecessor. In ISC BIND9 this option is called
"ixfr-from-differences" and is known to be performant even for
million-rule DNS RPZ's with significant churn on a minute by minute
basis.
It is good operational practice to include test records in each DNS
RPZ to help that DNS RPZ's subscribers verify that response policy
rewriting is working. For example, a DNS RPZ might include a QNAME
policy rule for "BAD.EXAMPLE.COM" as well as a Response IP Address
policy rule for 192.0.2.1. A subscriber can verify the correctness
of their installation by querying for "BAD.EXAMPLE.COM", which does
not exist in real DNS. If an answer is received it will be from the
DNS RPZ. That answer's additional data section will usually contain
an SOA RR denoting the fully qualified name of the DNS RPZ itself.
8. Subscriber Behavior
RPZs MUST be primary or secondary zones at subscriber recursive
resolvers. They can be searched using only a recursive server's own
storage, because additional network transactions for DNS resolvers
would be unworkable for this purpose.
Response policy zones are loaded in the usual way. For primary zones
this may mean loading the contents of a local file into memory or
connecting to a database. For secondary zones this means
transferring the zone from the primary server using zone transfer
such as IXFR [RFC1995] or AXFR [RFC5936]. All secondary zone
transfer relationships SHOULD be protected with transaction
signatures (DNS TSIG) and real time change notification SHOULD be
enabled using the DNS NOTIFY protocol [RFC1996].
9. Implementation Considerations
DNS resolvers often have limited notion or no notion of a DNS zone or
zone file. They sometimes have special local zones, but generally
have no implementations of IXFR, AXFR, or NOTIFY. Therefore, an
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external module or service that maintains local copies of policy
zones can be useful.
RPZ checks can add significant processing and network costs to the
processing of recursive DNS requests. This is particularly true of
rules with NSDNAME and NSIP triggers. However, NSDNAME and NSIP
triggers can be more effective than QNAME and IP triggers, because
miscreants can more easily change their direct domain names and IP
addresses (which are detected by QNAME and IP triggers) than they can
change their NS names and addresses.
To minimize the costs associated with RPZ processing, implementations
MAY use various optimizations, some of which are described below.
9.1. Avoid Waiting for QNAME Recursion
When data already in the resolver's cache combined with the
precedence rules in Section 5 are sufficient to produce a final RPZ
result, an implementation MAY choose not to finish computing the
unmodified DNS response, thus avoiding unnecessary recursion. For
example, a matching Client IP Address trigger (Section 4.1) in the
first specified RPZ always provides the result regardless of what
might be learned by asking (recursing to) the authority for the
QNAME. Of course, if the data already in the cache are not
dispositive, then the implementation MUST use recursion. For example
a matching QNAME trigger (Section 4.2) in the second RPZ could
potentially be overruled by a Response IP Address trigger
(Section 4.3) in the first RPZ.
Implementations which include this optimization SHOULD provide a
configuration switch (for example, "qname-wait-recurse") to turn it
on and off. The default value of the switch MAY be on or off. Some
security implications of avoiding unnecessary recursion are
considered in Section 12.4.
Implementations MAY cause the LOCAL-DATA-OR-DISABLED override
(described in Section 6.1) to imply not waiting for recursion, or to
require that optimization.
9.2. Check Parents Domains versus Zone Cuts
In principle, the NSDNAME and NSIP processes of following the data
path toward the root look for NS, A, and AAAA RRsets only in the
apexes of zones. In practice, DNS resolvers often do not know which
domain name parent-child relationships reflect zone delegations, and
so RPZ implementations generally iteratively remove labels from the
left end of the domain name to request NS RRsets from their caches or
authoritative servers for each ancestor of the queried name. The
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cost of looking for an NS RRset for a domain name that is not at a
delegation point is usually only asking for the RRset and receiving
only an SOA from the cache.
9.3. Abbreviate the Data Path for NSDNAME and NSIP Checks
The process of traversing the data path from the level nearest the
queried record to the top (root domain) level to check NSIP or
NSDNAME triggers can be expensive. There are many NS records for the
top level domains and the root, but checking them is rarely
worthwhile because the root and TLDs are rarely used as NSIP or
NSDNAME triggers. Some RPZ implementations have a "min-ns-dots"
parameter that stops NSDNAME and NSIP checking short of the root
zone, by not checking the name servers for domains whose names
contain fewer than the configured minimum number of dots.
9.4. Use Glue for NSDNAME and NSIP Checks
Some implementations of DNS RPZ attempt to exhaustively discover all
ancestral zone cuts above the query name and learn the NS RRset at
the apex of each delegated zone. Other implementations use only the
information which has naturally come into their caches, which often
includes only delegations and glue, that is name server names and
addresses sent by servers for the parent zone and not by those for
the zone itself. Both of these tactics have shortcomings, but the
more accurate tactic of checking delegations and glue as well as
authoritative NS, A, and AAAA RRsets is generally considered too
expensive.
To limit the costs of checking NSDNAME and NSIP triggers, an
implementation MAY have settings (for example "nsdname-wait-
recurse no" or "nsip-wait-recurse no") which cause the resolver to
use only the NS RRsets or only the A and AAAA RRsets already in the
cache. With these settings, the resolver will not recurse if there
are relevant cached entries (including negative entries) when
processing NSDNAME or NSIP triggers, respectively.
Note that using only already-cached instead of current and complete
delegation, glue, and authoritative data can cause inconsistent RPZ
behavior. Most recursive DNS server implementations do not renew
delegation and glue RRsets when their TTLs expire if authoritative
data are available in the cache. Requests by the RPZ part of a
recursive server for NS RRsets and associated A and AAAA RRsets will
often initially receive only delegations and glue from the cache. As
time passes and TTLs expire, those requests tend to be answered less
with delegations and glue and more with authoritative RRsets. If and
when sufficient RRsets expire from the cache, the cycle repeats with
RPZ once again relying more on delegations and glue. Thus, if the
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glue and authoritative RRsets differ, then RPZ installations that use
whichever is available in the cache can produce inconsistent results.
This is not an argument for NSDNAME or NSIP triggers to not use glue,
because as discussed in Section 12.5, delegations and glue NS, A, and
AAAA RRsets for miscreant domains are often more revealing and
effective than their authoritative RRsets.
9.5. Reduce Zone Size using Implied Rules
In 2016, an RPZ zone of QNAME rules for many miscreant domain names
contained 200 MBytes of data representing almost 8 million rules. In
such an RPZ, many of the listed domain names are as undesirable as
DNS servers as they are as mail senders, and so might reasonably have
NSDNAME rules in addition to their QNAME rules. However, instead of
paying the costs of doubling that RPZ zone to 16 million rules and
400 MBytes, an RPZ implementations MAY offer a per-zone setting (for
example "qname-as-ns yes") which causes the implementation to act as
if for every rule with the trigger "EXAMPLE.COM", there is another
rule in the RPZ with the trigger "EXAMPLE.COM.RPZ-NSDNAME" and the
same policy action.
An implementation MAY also offer a setting (for example "ip-as-
ns yes") which similarly generates NSIP rules based on Response IP
Address rules. Given an RPZ rule with a trigger of
"24.0.2.0.192.rpz-ip", act as if the RPZ also contains another rule
with the trigger "24.0.2.0.192.rpz-nsip" and the same policy action.
10. History and Evolution
An early description of RPZ can be found in a technical note from
Internet Systems Consortium [ISC-RPZ]. RPZ was also described in
2010 in chapter 6 of the "BIND 9 Administrator Reference Manual"
[ISC-ARM].
RPZ was designed by Paul Vixie and Vernon Schryver in 2009. The
initial implementation and first patch adding it to BIND were written
by Vernon Schryver in late 2009. Patches for various versions of
BIND9 including 9.4, 9.6, and 9.7 were distributed from FTP servers
at redbarn.org and rhyolite.com starting in 2010.
If all RPZ triggers and actions had been foreseen at the start in
2009, they would probably have been encoded differently. Instead RPZ
grew incrementally, and upward compatibility required support of the
original encodings. The initial specification or Format 1 contained
only QNAME triggers. Changes for Format 2 included adding triggers
based on response contents (Response IP Address), the targets of NS
records (NSDNAME), and contents of A and AAAA records that resolve NS
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records (NSIP). Format 3 included "rpz-passthru" for the PASSTHRU
action and wildcards in Local Data CNAME targets to synthesize
responses dynamically based on the query domain.
Today, with a number of commercial RPZ providers with many users and
no functional problems with the encodings, any lack of aesthetic
appeal is balanced by the ever increasing weight of the installed
base. For example, it is impossible to replace the original QNAME
trigger encoding NXDOMAIN and NODATA policy action encodings with
encodings that involve rpz-* pseudo-TLDs at RPZ providers without
breaking the many existing RPZ subscriber installations. The
original, deprecated PASSTHRU encoding of a CNAME pointing to the
trigger QNAME might still be in use in local, private policy zones,
and so it is still recognized by RPZ subscriber implementations as of
2016.
The initial idea for RPZ was only to deny the existence of
objectionable domain names, and so there existed only QNAME triggers
and NXDOMAIN actions. Given that single kind of trigger, encoding it
as the owner name of a policy record was clearly best. A CNAME
pointing to the root domain (.) is a legal and valid but not
generally useful record, and so that became the encoding for the
NXDOMAIN action. The encoding of the NODATA action as "CNAME *."
followed similar reasoning. The addition of more triggers and more
actions required a more general scheme, and so newer actions are
encoded as CNAMEs with targets in bogus TLDs whose names start with
"rpz-". Newer triggers are owner names containing DNS labels that
start with "rpz-".
It would have been better if all actions had been encoded as
subdomains of the invalid "*" top level domain, for example
"drop.rpz.*" and "passthru.rpz.*" and even "nxdomain.rpz.*". "rpz"
would always be the second level domain and the action would be
specified by the third level domain. That would have polluted only a
single bogus TLD. Perhaps future versions of RPZ that define new
actions will define those new actions using that scheme, and will use
that scheme to redefine existing actions while deprecating but
retaining the old definitions.
11. IANA Considerations
No actions are required from IANA as result of the publication of
this document.
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12. Security Considerations
12.1. DNS Data Security
RPZ is a mechanism for providing "untruthful" DNS results to
consenting stub resolvers from recursive servers. Nevertheless, RPZ
does not exacerbate the existing vulnerability of DNS servers to
falsified DNS data; it merely formalizes and facilitates modifying
DNS data on its way from DNS authority servers to clients. Note that
a non-consenting stub resolver can simply opt out by using a
different recursive name server.
12.2. DNSSEC
The use of DNSSEC (see [RFC4033] and [RFC4034]) prevents the
acceptance by clients of such RPZ-induced changes to DNS data.
Therefore, by default, DNS resolvers using RPZ avoid modifying DNS
results when DNSSEC signatures are available and are requested by the
DNS client. However, when the common "break-dnssec" configuration
setting is used, RPZ-using resolvers rewrite responses even in that
case. They omit DNSSEC RRsets, because the modified responses cannot
be verified by DNSSEC signatures. This renders the results invalid
according to DNSSEC. In such a case, a querying client which checks
DNSSEC reacts as though it has been subjected to a data poisoning
attack and rejects the data. That reaction could entail expensive
bypass operations.
12.3. TSIG
The policy zones might be considered sensitive, because they contain
information about miscreants. Like other DNS zones in most
situations, RPZs are transferred from publishers to subscribers as
cleartext vulnerable to observation. DNS zones are vulnerable to
forgery or modification if TSIG transaction signatures [RFC2845] are
not used, and so TSIG SHOULD be used to authenticate RPZ contents and
protect them from modification.
12.4. Counterintelligence
Recursive servers using RPZ can be optimized to avoid completing
recursion if a policy rule provides a rewritten answer without
needing this recursion (Section 9.1). However, the use of such an
optimization allows miscreants to infer whether RPZ is in use and
whether their RRs are listed, simply by observing requests sent to
their own authority servers. Therefore, implementations that provide
this optimization SHOULD allow it to be disabled.
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12.5. NSIP, NSDNAME, Glue, and Authoritative RRsets
Apex ("below the cut") or authoritative NS, A, and AAAA RRsets
containing the name server names and addresses for a zone often do
not exactly match glue from the parent zone. Delegation and glue
records or "above the cut" records from the parent zone are often
incomplete or out of date compared to the NS, A, and AAAA RRs from
authoritative servers for the zone itself. Regardless of any lack of
completeness or accuracy, the delegation and glue records for a
functioning domain are sufficently accurate for the domain to be
resolvable: the authoritative servers for a zone cannot be found
without a delegation and (if necessary) some glue. On the other
hand, a domain with entirely broken authoritative name server data
can largely work.
The authoritative NS data for miscreant domains is often fanciful or
even unavailable. For example, an authoritative NS RRset consisting
of "NS ." is popular, because while it is wrong, it betrays nothing
in particular about the miscreant who owns the domain.
Thus delegations and glue are generally best for detecting and
rewriting miscreant DNS data, if resource constraints prohibit
checking both above-delegation and below-delegation NS and glue.
13. Acknowledgements
The authors gratefully acknowledge the substantial contributed
material and editorial scrutiny of Anne Bennett. She directed the
reorganization and clarification of the entire document.
Eric Ziegast, Jeroen Massar, Ben April, Ray Bellis and Mukund
Sivaraman provided improvements to the document and caught errors.
14. References
14.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<http://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.
[RFC1995] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
DOI 10.17487/RFC1995, August 1996,
<http://www.rfc-editor.org/info/rfc1995>.
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[RFC1996] Vixie, P., "A Mechanism for Prompt Notification of Zone
Changes (DNS NOTIFY)", RFC 1996, DOI 10.17487/RFC1996,
August 1996, <http://www.rfc-editor.org/info/rfc1996>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, DOI 10.17487/RFC2136, April 1997,
<http://www.rfc-editor.org/info/rfc2136>.
[RFC2845] Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
Wellington, "Secret Key Transaction Authentication for DNS
(TSIG)", RFC 2845, DOI 10.17487/RFC2845, May 2000,
<http://www.rfc-editor.org/info/rfc2845>.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<http://www.rfc-editor.org/info/rfc4033>.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, DOI 10.17487/RFC4034, March 2005,
<http://www.rfc-editor.org/info/rfc4034>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <http://www.rfc-editor.org/info/rfc4291>.
[RFC5936] Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
(AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,
<http://www.rfc-editor.org/info/rfc5936>.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952,
DOI 10.17487/RFC5952, August 2010,
<http://www.rfc-editor.org/info/rfc5952>.
14.2. Informative References
[ISC-ARM] Internet Systems Consortium, "BIND 9 Administrator
Reference Manual,
https://ftp.isc.org/isc/bind9/cur/9.10/doc/arm/
Bv9ARM.ch06.html#rpz", 2016.
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[ISC-RPZ] Vixie, P. and V. Schryver, "DNS Response Policy Zones (DNS
RPZ, Format 3), https://ftp.isc.org/isc/dnsrpz/isc-tn-
2010-1.txt", 2010.
[ISC-RRL] Vixie, P. and V. Schryver, "DNS Response Rate Limiting
(DNS RRL), https://ftp.isc.org/isc/pubs/tn/isc-tn-
2012-1.txt", 2012.
Appendix A. Examples
An existing data feed capable of producing an RHSBL can be trivially
used to generate a DNS RPZ. If the desired policy is to alias
targeted domains to a local walled garden, then for each domain name,
generate the following records, one for the name itself and perhaps
also one for its subdomains:
bad.example.com CNAME walled-garden.example.net.
*.bad.example.com CNAME walled-garden.example.net.
If it is desirable to return NXDOMAIN for each domain (and its
subdomains in this example), try this:
bad.example.com CNAME .
*.bad.example.com CNAME .
Try this if there are walled gardens for mail versus everything else:
bad.example.com MX 0 wgmail.example.net.
bad.example.com A 192.0.2.66
*.bad.example.com MX 0 wgmail.example.net.
*.bad.example.com A 192.0.2.66
An extended example follows:
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$ORIGIN rpz.example.net.
$TTL 1H
@ SOA LOCALHOST. named-mgr.example.net. (
1 1h 15m 30d 2h) NS LOCALHOST.
; QNAME policy records.
; There are no periods (.) after the relative owner names.
nxdomain.example.com CNAME . ; NXDOMAIN policy
nodata.example.com CNAME *. ; NODATA policy
; Redirect to walled garden
bad.example.com A 10.0.0.1
AAAA 2001:db8::1
; Rewrite all names inside "AZONE.EXAMPLE.COM"
; except "OK.AZONE.EXAMPLE.COM"
*.azone.example.com CNAME garden.example.net.
ok.azone.example.com CNAME rpz-passthru.
; Redirect "BZONE.EXAMPLE.COM" and "X.BZONE.EXAMPLE.COM"
; to "BZONE.EXAMPLE.COM.GARDEN.EXAMPLE.NET" and
; "X.BZONE.EXAMPLE.COM.GARDEN.EXAMPLE.NET", respectively.
bzone.example.com CNAME *.garden.example.net.
*.bzone.example.com CNAME *.garden.example.net.
; Rewrite all answers containing addresses in 192.0.2.0/24,
; except 192.0.2.1
24.0.2.0.192.rpz-ip CNAME .
32.1.2.0.192.rpz-ip CNAME rpz-passthru.
; Rewrite to NXDOMAIN all responses for domains for which
; "NS.EXAMPLE.COM" is an authoritative DNS server for that domain
; or any of its ancestors, or that have an authoritative server
; in 2001:db8::/32
ns.example.com.rpz-nsdname CNAME .
32.zz.db8.2001.rpz-nsip CNAME .
; Local Data can include many record types
25.128.2.0.192.rpz-ip A 172.16.0.1
25.128.2.0.192.rpz-ip A 172.16.0.2
25.128.2.0.192.rpz-ip A 172.16.0.3
25.128.2.0.192.rpz-ip MX 10 mx1.example.com
25.128.2.0.192.rpz-ip MX 20 mx2.example.com
25.128.2.0.192.rpz-ip TXT "Contact Central Services"
25.128.2.0.192.rpz-ip TXT "Your system is infected."
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Authors' Addresses
Paul Vixie
Farsight Security, Inc.
Email: paul@redbarn.org
Vernon Schryver
Rhyolite Software, LLC
Email: vjs@rhyolite.com
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